The Sandwich Plate System (SPS) is created by bonding two steel plates together with an elastomer core that is injected into a cavity formed by the steel plates and perimeter bars. The result is a stiffer and lighter panel that can be used for plate-like structures such as bridge decks, stadium risers or ship decks. For more versatility, the effects of welding post-injection to the SPS panels were investigated. Three post-injection welded joints were tested to determine fatigue resistance and the effects of cyclic loading on the localized debonding of the heat affected zone at the post-injection welded joint of a SPS bridge deck. Seven panels containing one of three post-injection weld configurations were investigated. Each panel was fatigue tested to ten million cycles or until failure, by applying remote bending to the post-injection welded joint. Experimental deflections and strains were compared to finite element analyses. Fatigue-life predictions were made using code based S-N curves, and a relatively new mesh-insensitive structural stress method with a master S-N curve approach. The post-injection welded joint demonstrated good fatigue resistance to recommended AASHTO loading when shims were used under the middle support to offset the camber in the SPS panels. It was also found that stresses caused by draw down of the camber had an adverse affect on the post-injection welded joint and greatly reduced its fatigue resistance. / Master of Science